The present invention relates to a synthetic fiber rope, preferably of aromatic polyamide.
Especially in materials handling technology, for example on elevators, in crane construction, and in mining, ropes are an important element of machinery and subject to heavy use. An especially complex aspect is the loading of driven or over pulleys deflected ropes, for example, as they are used in elevator construction.
In conventional elevator installations the car sling of a car, which is moved in an elevator hoistway, and a counterweight are connected together by a steel rope. To raise and lower the car and the counterweight, the rope runs over a traction sheave that is driven by a drive motor. The drive torque is transferred by friction to the section of the rope which at any moment is lying in the angle of wrap. At this point the rope is subjected to high transverse forces. As the loaded rope is reversed by passing over the traction sheave, the strands move relative to each other to compensate for differences in tensile stress. The same refers to ropes wound on drums as they are used in elevators or cranes.
On elevator installations the lengths of rope needed are large, and considerations of energy lead to the demand for smallest possible masses. High-tensile synthetic fiber ropes, for example of aromatic polyamides or aramides with highly oriented molecule chains, fulfil these requirements better than steel ropes.
By comparison with conventional steel ropes of the same cross sectional area, ropes constructed of aramide fibers have a substantially higher lifting capacity and only between one fifth and one sixth of the specific gravity. In contrast to steel, however, the atomic structure of aramide fiber causes it to have a low ultimate elongation and a low shear strength.
Consequently, so that the aramide fibers are subjected to the smallest possible transverse stresses as they pass over the traction sheave, there is proposed in European patent document EP 0 672 781 A1, for example, an aramide fiber rope suitable for use as a traction rope. Between the outermost and inner layers of strands there is an intersheath which prevents contact between the strands of different layers and thereby reduces the wear due to their rubbing against each other. The previously known aramide rope described so far has satisfactory values of service life, resistance to abrasion, and fatigue strength under reversed bending stresses; however, it has been established that due to the parallel lay there is a possibility that in the permanently loaded traction rope, an inner torque acts over a section of rope beginning at the traction sheave, and as it passes over the traction sheave the section twists or untwists about its longitudinal axis. As a consequence of the resulting stress, changes in the structure can occur, which then lead to excessive length of individual outermost strands. The excessive lengths are transported within the rope in repeated passages of the rope over the traction sheave. Such a change in the structure of the rope is undesirable because it could lead to a reduction in the breaking load of the rope or even to failure of the rope.